Evaluation of the relaxant effects of SCA40, a novel charybdotoxin‐sensitive potassium channel opener, in guinea‐pig isolated trachealis

Abstract

1 Experiments have been performed in order to analyse the mechanism whereby SCA40, a new imidazo[1,2-a]pyrazine derivative relaxes airway smooth muscle.2 SCA40 (0.01-10 gM) caused a complete and concentration-dependent relaxation of guinea-pig isolated trachea contracted with 20 mM KC1 but failed to inhibit completely the spasmogenic effects of 80mM KC1.3 Quinine (30 gM) antagonized the relaxant activity of SCA40 in 20 mM KCl-contracted guinea-pig isolated trachea. The ATP-sensitive K+-channel blocker, glibenclamide (3 gM), did not antagonize the relaxant activity of SCA40 in either 20 mM KC1 or 1 JLM carbachol-contracted isolated trachea. 4 SCA40 (0.01-10tM) and isoprenaline (0.1 nM-10 M) caused a complete and concentrationdependent relaxation of guinea-pig isolated trachea contracted with carbachol 1 ELM. 5 The large-conductance Ca2+-activated K+-channel blocker, charybdotoxin (60-180 nM), non-competitively antagonized the relaxant activity of isoprenaline on 1 gM carbachol-contracted trachea. The inhibition was characterized by rightward shifts of the isoprenaline concentration-relaxation curves with depression of their maxima. 6 The relaxant activity of SCA40 in 1 gM carbachol-contracted trachea was antagonized by charybdotoxin (60-600 nM) in an apparently competitive manner. The concentration-relaxation curves to SCA40 were shifted to the right with no significant alteration in the maximum response. 7 It is concluded that SCA40 is a novel potassium channel opener which is a potent relaxant of guinea-pig airway smooth muscle in vitro. The relaxant activity of SCA40 does not involve ATPsensitive K+-channels but rather large-conductance Ca2'-activated K+-channels or other charybdotoxinsensitive K+-channels.

A comparison of the effects of SCA40, NS 004 and NS 1619 on large conductance Ca2+‐activated K+ channels in bovine tracheal smooth muscle cells in culture

“…This conclusion is in contrast to that of Laurent et al (1993) but is entirely consistent with the findings of Cook et al (1995).…”

“…(Laurent et al, 1993). ductance (240 pS), selectivity for K+, dependence on inIt is conceivable then that a species difference in the channel tracellular Ca2+ concentration and sensitivity to the specific structure could explain the present negative findings, and that BKca channel blocker, iberiotoxin (Galvez et at., 1990). The BKca channels on guinea-pig trachealis cells possess a represence of a 249 pS charybdotoxin-sensitive BKca channel in cognition site for SCA4O which is absent on BKca channels in acutely dissociated BTSM cells has been reported previously bovine trachealis.…”

“…(1991) Cook et at. (1995) who examined the pharmacology of SCA40 in guinea-pig isolated trachealis muscle and found that although its relaxant effects were attenuated in K+-rich Krebs solution and by ChTx (confirming the original observations of Laurent et al, 1993), this was due to the fact that the high K+ and ChTX treatments in themselves promoted Ca2" entry (and therefore contraction), and that this action produced a functional antagonism of the SCA40-induced relaxation. On the basis of these and other data, Cook et al (1995) concluded that the smooth muscle relaxant action of SCA40 was most likely to arise from the potent inhibition by this compound of phosphodiesterase type III.…”

“…The most potent of these compounds, SCA40, has been reported to exert its relaxant effects by opening BKca channels, a conclusion based on the sensitivity of the SCA40-induced relaxation to the BKca channel blocker, charybdotoxin (ChTX), and on the observation that the relaxation was attenuated when 80 mM KCl was used as the spasmogen (Laurent et al, 1993). However, the assertion that SCA40 acts via direct opening of BKCa channels has recently been disputed (Cook et al, 1995).…”

Effects of some K+‐channel inhibitors on the electrical behaviour of guinea‐pig isolated trachealis and on its responses to spasmogenic drugs

“…in determining the sensitivity and responsiveness of airways smooth muscle to relaxant drugs has been investigated with inhibitors selective for BKca such as ChTX and IbTX (Jones et al, 1990;1993;Murray et al, 1991;Huang et al, 1993;Laurent et al, 1993;Cook et al, 1995).…”